In the fabrication of semiconductor devices, one of the more esoteric problems which arises is in providing a contact to a semiconductor substrate which is compatible with both the substrate and the metallurgy which interconnects with other contacts on the same substrate. This is no simple matter in view of the fact that factors like the kind of semiconductor, the doping of the semiconductor and the type of metals involved may influence whether the resulting contact is ohmic or rectifying.
U.S. Pat. No. 3,987,480 filed May 17, 1974 shows a gallium arsenide substrate with an ohmic contact to a high resistivity region. It shows tantalum and germanium being coevaporated on gallium arsenide and being heated between 400.degree. and 600.degree. C. in a neutral or reducing atmosphere for 10 to 30 minutes. Under these circumstances, a germanide of tantalum is not formed in that the temperature utilized is insufficient to form the first-to-form germanide of tantalum.
U.S. Pat. No. 4,011,583 filed May 27, 1976 shows an ohmic contact to n-type III-V semiconductors in which a metallurgical combination including germanium and palladium is formed on the semiconductor surface either in the form of an alloy or discrete layers. The resulting structure is heated for five minutes to two hours in a temperature range of 200.degree.-700.degree. C. The reference suggests that palladium rich compounds such as Pd.sub.2 Ge are formed. Palladium is not, however, a refractory metal and the resulting ohmic contact has a contact resistance higher than that of the present invention.
U.S. Pat. 4,188,710 filed Aug. 11, 1978 shows a gallium arsenide substrate with a layer of epitaxial germanium formed thereon. After this, a layer of tungsten, tantalum or molybdenum is formed over the germanium layer. The structure is annealed at a temperature of 400.degree. C. to 500.degree. C. for thirty minutes. In this reference, no germanide layer whatsoever is formed in view of the annealing temperatures used. Using the technique of the patent, contact resistance of the order of 10.sup.-4 ohm cm.sup.2 are obtainable as opposed to the contact resistance of contacts made in accordance with the teaching of the present invention which are approximately two orders of magnitude better.
It is, therefore, an object of the present invention to provide a high-temperature, ohmic contact to binary and ternary compounds of gallium and arsenic. Another object is to provide an ohmic contact to n-type GaAs which is highly reliable and maintains stable characteristics under high temperature processes.
Still another object is to provide an ohmic contact wherein the contact resistance is several orders of magnitude smaller than prior art contacts to n-type GaAs.
Yet another object is to provide an ohmic contact to n-type gallium arsenide wherein the metals used cause the underlying gallium arsenide to become more n-type as result of reacting preferentially with gallium thereby creating excess gallium vacancies and permitting heavier doping with germanium.
Still another object is to provide a contact forming process to III-V compound semiconductors by preferentially forming germanides of certain refractory metals by sintering at a temperature and a time sufficient to form the first-to-form germanides of such metals.